One of a large family of carotenoids (found in carrots), beta carotene is the main one added to supplements. In proper dosing it can be helpful, and higher doses (to a point) are associated with increased longetivity.1
Because beta carotene is part of a larger "family," however, taking very high doses of only one type can actually suppress the others and become problematic. For example, taking 25,000 units a day was associated with an increased risk of lung cancer. So more is not always better!
1Ray AL, Semba RD, et al, "Low Serum Selenium and Total Carotenoids Predict Mortality among Older Women Living in the Community: The Women’s Health and Aging Studies," J Nutr, 2006; 136(1): 172-6